1. Field of the Technology
The technology presented herein relates to a semiconductor laser device and a method for manufacturing the same, and more specifically, to a semiconductor laser device used for an OA apparatus, such as a copying machine and a laser beam printer, and an optical information processing apparatus, such as an optical fiber communication system, an optical measuring system and an optical disc, and to a method for manufacturing such a semiconductor laser device.
2. Description of the Related Art
In recent years, with respect to OA apparatuses and optical information processing apparatuses provided with a semiconductor laser device, there have been strong demands for high-speed operations and large-size capacities for information processing, and for this reason, the application of a multi-beam semiconductor laser element (hereinafter, referred to also as a multi laser or a laser chip) capable of irradiating a plurality of laser beams has been proposed (refer to, for example, JP-A No. 2002-374029, JP-A No. 2004-356586 and JP-A No. 2005-45146).
The multi laser is provided with a plurality of light emitting units formed on a semiconductor substrate in the form of stripes electrically separated from one another respectively, and a first electrode and a second electrode formed on a surface of each of the light emitting units and a back surface of the substrate on the side opposite to the light emitting units respectively. Moreover, the multi laser is assembled on a submount (referred to also as a heat sink) by using an electrode structure and an packaging system, as shown in FIGS. 6 to 10, so as to allow the respective light emitting units to be driven independently, and then mounted on a package.
The submount is mainly used as a heat radiating member that absorbs heat generated by a laser chip, and releases the heat externally. Here, with respect to the package on which the laser chip is mounted, since it is difficult to call for its mount portion to have flatness against irregularities in the order of several micrometers, and since it is also difficult to adopt a member that is unlikely to transfer a thermal strain to the laser chip (i.e., member having virtually the same thermal expansion coefficient as that of the material of the laser chip) as one portion thereof, the laser chip is generally mounted on the package through the submount. Therefore, the submount is generally required to have a superior heat radiating property, low costs and a thermal expansion coefficient close to that of the material for the laser chip, and generally made from a material such as Si, AlN and SiC.
FIG. 6 shows a state in which a multi laser is packaged on a submount by using a so-called junction-up system. In this case, a multi laser 110 has a structure in which first electrodes 113 are formed on the surfaces of a plurality of light emitting units 112 that are formed on a semiconductor substrate 111 in the form of stripes, with a second electrode 114 being formed on the entire back surface of the semiconductor substrate 111. Moreover, the submount 116 is provided with a connection electrode 117 formed on the entire surface (upper face) thereof.
In the junction-up system, the second electrode 114 on the semiconductor substrate 111 side is electrically connected to the connection electrode 117 on the submount 116 through a solder material. Moreover, the respective first electrodes 113 are electrically connected to an external circuit (not shown) by using wire bonding through wires 118.
FIG. 7 also shows a state in which a multi laser is packaged on a submount by using the junction-up system. In this case, a multi laser 210 has a structure in which a plurality of bonding pads 215 corresponding to a plurality of first electrodes 213 are formed on the two sides of a plurality of light emitting units 212 having the form of stripes, with connecting wires 215a to be used for connecting the respective bonding pads 215 to the respective first electrodes 213 being formed thereon. Among these connecting wires 215a, an insulating film is formed between the first electrode 213 and the connecting electrode 215a that are not to be connected. Moreover, the respective bonding pads 215 are electrically connected to an external circuit (not shown) by using wire bonding through wires 118. Here, in FIG. 7, reference numeral 214 denotes a second electrode, and the same constituent elements as those shown in FIG. 6 are denoted by the same reference numerals.
FIG. 8 shows a state in which a multi laser is packaged on a submount by using a so-called junction-down system. In FIG. 8, the same constituent elements as those shown in FIG. 6 are denoted by the same reference numerals.
In this case, the multi laser 110 explained in FIG. 6 is used. On the other hand, a submount 316 has a structure in which a plurality of connection electrodes 317 are formed on a packaging area on its surface in the form of stripes with the same interval as the interval with which a plurality of first electrodes 113 of the multi laser 110 are arranged side by side, while a plurality of bonding pads 318 and connection wires 319 used for connecting the respective bonding pads 318 to the respective connection electrodes 317 are formed on areas other than the packaging area.
In the junction-down system, the respective first electrodes 113 of the multi laser 110 are electrically connected to the respective connection electrodes 317 through a solder material. Moreover, the respective bonding pads 318 are electrically connected to an external circuit (not shown) by using wire bonding through wires 118.
In the case of the junction-up system shown in FIG. 6, since the plurality of first electrodes 113 are respectively wire-bonded, the width of the first electrodes 113 is required to have such a dimension as not to allow a wire bonding ball to protrude therefrom. For example, supposing that the diameter of the wire 118 is set to 25 μm, which is a general value, since the size of the wire bonding ball is 75 μm or more that is three to four times wider, the width of the first electrodes 113 needs to be set to at least 75 μm. For example, in the case of a laser beam printer, in order to achieve high speed operations, a semiconductor laser element having a narrow light emission-point interval with more light-emission points has been demanded; however, the junction-up system of FIG. 6 in which the first electrodes 113 are limited by the size of the wire bonding ball, as described above, fails to provide an optimal packaging system.
In the case of the junction-up system shown in FIG. 7, since no wire bonding is directly carried out on the first electrodes 213, the light emission-point interval can be made narrower than that of FIG. 6 to, for example, 14 μm; however, since the area used for forming the bonding pads 215 needs to be ensured, the area that does not devote to light emission increases, resulting in an expansion in the chip size of the multi laser.
Moreover, the junction-up system, shown in FIGS. 6 and 7, is disadvantageous in efficiently releasing heat generated from the light emitting unit of the multi laser to the submount, and consequently tends to cause degradation of characteristics as well as degradation of reliability, failing to provide a desirable packaging system.
In the case of the junction-down system shown in FIG. 8, since the multi laser 110 is packaged with the light emitting unit 112 being located closely to the submount 316, this packaging system efficiently releases heat generated from the light emitting unit to the submount. However, upon connecting a plurality of the first electrodes 113 to a plurality of the connection electrodes 317, since it is necessary to melt the solder material and also to carry out the anchoring process, without directly viewing the connection electrodes 317 of the submount 316 and the first electrodes 113 of the multi laser 110, with an electrical separation between the connection electrodes 317 and the first electrodes 113 that are not to be electrically connected to each other being maintained; therefore, this system causes a difficult packaging process, with high positional precision being required. In particular, upon packaging a multi laser having a narrow light emission-point interval (for example, light emission-point interval of 50 μm or less), a more difficult packaging process is required.
When the first electrodes 113 and the connection electrodes 317 are connected to each other with poor positional precision, protruded solder material between the first electrodes 113 and the connection electrodes 317 and positional deviations in die bonding precision tend to occur, resulting in a current leak to subsequently cause degradation of functions and a malfunction in an electronic apparatus provided with the multi laser 110.